The present invention relates to a method of achieving improved quality estimation when detecting information frames, and particularly when detecting erroneous speech frames in conjunction with channel decoding and subsequent speech decoding in the receiver of a radio communication system which operates, for instance, with time division multiple access (so-called TDMA). The inventive method, however, can also be used with frequency division radio systems (FDMA). The invention also relates to an arrangement in a radio station using the method.
In a radio system which operates with time division multiple access (e.g. TDMA), data messages and control messages are transmitted in bursts over certain time slots between a primary station (base station) and one or more secondary stations (mobiles).
The primary and the secondary stations both have a transmitter and a receiver side. The transmitter side includes a speech coder, channel coder and a modulator. The receiver side includes corresponding units, namely a demodulator, channel decoder and speech decoder.
Speech to be transmitted from, for instance, a secondary station (mobile) to a base station is speech-coded in the transmission side of the secondary station and is divided into speech frames prior to channel coding and transmission in the form of bursts in accordance with the access method (TDMA) concerned.
For instance, the speech coded signals may be divided in speech frames (which are not the same as the TDMA-frames) with coded speech samples and transmitted in accordance with a given access method (TDMA). For instance, the speech can be divided in speech frames each of 20 ms. This gives 160 samples at a sampling rate of 8 kHz, which in turn gives 260 bits/speech frames at a speech coding rate of 13 kbit/s.
The speech frames are coded in the channel coder in accordance with a given code, which may be a block code or a convolution code. Some of the bits in a speech frame are more sensitive, i.e. carry more important information and it is thus more important that these bits can be received correctly.
These more sensitive bits can be protected, by adding redundant parity bits in the channel coder. These redundant bits can be used for error detection and/or error correction of bit errors later-on in the receiver. The so-called bit error rate (i.e. quality) of a received message, e.g. a speech frame, may also be estimated in the receiver. When a bit-containing frame is received, the channel decoder uses the redundant bits to correct the most sensitive bits. If it is not possible to correct bit errors, it is necessary to indicate that the whole bit contents of the speech frame (or a part of a speech frame) is bad, i.e. erroneous.
When decoding speech in the receiver (speech synthesis), a bad frame can result in greatly impaired speech. The most sensitive, or susceptible, bits belong to such frame parameters as amplification and spectral distribution. When errors occur in bits which represent these parameters, the decoded and outgoing speech will be heavily distorted by slamming sound and disturbing noise.
One method of improving the subjective quality in a digital cellular system in which, for instance, fading distorts a complete speech frame, is to take action so as to reduce the speech distortion. One example of such action is to repeat the previously received error-free speech frame parameters and/or introduce limitations of the speech frame parameters.
To the same end, it is also known to introduce a so-called BFI (Bad Frame Indicator) into the channel decoder of the cellular radio system GSM (Global System for Mobile Communication). This gives an indication in the form of a binary signal to the speech decoder on the receiver side, which denotes whether a frame error has occurred or not.
In order to achieve good properties regarding the aforesaid known measures of improving the subjective speech quality, it is necessary to achieve good and accurate estimation of the quality of the speech frame data received. For instance, the error indication given by the bad frame indicator, BFI, used in the GSM-system is too coarse, since the BFI only gives a logic "1" for an error-free frame and a logic "0" for a bad or erroneous frame. Frame detection which is more bit selective, i.e. detection which indicates more precisely those parts of a frame which contain erroneous bits, would greatly improve the speech quality. Furthermore, the BFI itself may be erroneous. Error-free frames which are detected as being erroneous, and similarly non-detected erroneous frames, will impair the possibilities of taking error-correcting measures in the subsequent speech decoding process (the speech synthesis).
The aforesaid earlier known error detecting block code necessitates the addition of redundant bits to the speech data bits, which means that the bit rate must be increased. When the number of redundant bits is reduced, the block code becomes too weak to provide satisfactory error detection.
Another known method, on which the present method is based, involves the use of so-called soft information in frame error indicating processes. These values are already available to the detection process, and hence no additional parameters or bits which are able to influence the bit rate, such as when using block codes are inserted. However, the sole use of soft information in the error-indicating process, as is earlier known, does not provide the improvement that is provided with the use of block codes. The correlation between soft information and bit errors is often not sufficiently high for the frames received. Furthermore, the correlation varies with different types of channels. The metrics from a so-called Viterbi decoder in the receiver constitute an example of soft information.